1. Field of the Invention
This invention relates to a liquid crystal optical modulation element using a ferroelectric.
2. Description of the Prior Art
Liquid crystal optical modulation elements using nematic liquid crystals or smectic liquid crystals have been known in the past. Liquid crystal optical modulation elements (i.e., modulators) using nematic liquid crystals include those which use the guest-host effect (Japanese Pat. No. 653,723), those which utilize a dynamic scattering mode (DSM: Japanese patent publication No. 21,729/1970), those which utilize a twist structure molecular orientation (Japanese Patent Laid-Open No. 11,737/1972), and so forth. Liquid crystal optical modulation elements using smectic liquid crystals include those which utilize a thermo-optical effect (F. J. Kahn: Appln. Phys. Lett. 22, 111 (1973). However, none of these liquid crystal elements are free from the problem of a low response speed.
To solve this problem of the low response speed, an optical modulation system having a quick response speed which makes use of a ferroelectric liquid crystal layer as been proposed (European Patent Laid-Open No. 32,362). This liquid crystal layer exhibiting a ferroelectricity was announced by R. B. Meyer et al (R. B. Meyer et al: J. Physics 36 L-69 (1975)). It is believed that when a liquid crystal layer has a helical crystal structure and the liquid crystal molecules are arranged at an inclination to the helical axis, the central symmetry collapses, thereby producing the ferroelectricity. Chiral smectic C phase and chiral smectic H phase are known as liquid crystal phases having the structure described above, and several liquid crystal compounds exhibiting a ferroelectricity have been synthesized using the chiral smectic C and H phases (Ph. Martinot-Lagarde: J. Physique 37, C3-129 (1976)).
On the basis of the principle described above, the intensity of light can be modulated by changing the intensity and direction of the electric field. According to the above European Patent Laid-Open No. 32,362, the helical structure of the liquid crystal is loosened when the thickness d of the liquid crystal layer is sufficiently reduced so that two stable states of molecular orientation, that is, the initial orientation state and a state in which an electric field exceeding a certain critical value E.sub.c is applied, can be obtained. In accordance with this system, it has been reported that one stable state can be changed over to the other stable state by applying an electric field of approximately 10.sup.5 V/cm for a period of several micro-seconds.
However, in practice it is impossible on an industrial basis to reduce the gap between the liquid crystal layers sufficiently, the above European Patent Laid-Open No. 32,362 does not provide an orientation film between them. In order to orient the liquid crystal molecules without the orientation film, however, either a shearing stress or a magnetic field must be applied, as described in this prior art reference. Accordingly, this system can not be mass-produced, and hence has no practical value.